In the investment casting of high temperature casting materials to form parts of complex geometry with walls defining undercut recesses or spaces, such as, for example, complex ferrous impellers for use in gas turbines, it is common to first construct an aluminum hub mold and an aluminum single blade mold by machining solid aluminum. The molds are injected with melted wax to form a wax hub pattern and a wax pattern for each blade which has compound curves. Each of the wax blade patterns is positioned on the wax hub pattern using a fixture, and the wax blades are welded or fused by heat to the wax hub which also receives a preformed ceramic pour funnel.
The combined wax patterns and the pour funnel are then dipped into a ceramic slurry, removed from the slurry and coated with sand or vermiculite to form on the wax patterns a ceramic layer having permeability. The layer is dried, and the dipping, sanding and drying operations are repeated several times to create a multiple layer ceramic shell mold enclosing or encapsulating the combined wax patterns. The shell mold and wax patterns and the pour funnel are then placed within a kiln and fired to remove the wax and harden the ceramic shell mold and pour funnel.
A molten ferrous metal or high temperature casting material is poured into the shell mold, and after the material hardens, the shell mold is removed by destroying the mold and to form an impeller capable of withstanding high temperatures. Since all of the wax patterns and the shell mold are destroyed during the production of the impeller, there is substantial cost to produce a number of impellers, with the result that complex impellers produced by the above described investment casting method are not commonly used in the high volume production of gas turbines and turbochargers. Instead, such gas turbines and turbochargers use ferrous impellers with less efficient and simple blade geometry which may be simply cast without using the above investment casting method.
It is also known to cast or produce non-ferrous parts or impellers with either simple or complex blade geometry using a solid mold casting process. In this process, a flexible and resilient positive pattern is made of the part or impeller by placing a solid positive master pattern of the impeller into a suitable flask and then pouring a flexible and resilient material, such as silastic or platinum rubber material, over the master pattern. After the flexible material has cured, the solid master pattern of the impeller is removed from the flexible material to form a flexible mold with a reverse or negative cavity of the master pattern. A closed flask is then placed around the flexible mold of the master pattern, and a flexible and resilient curable material is poured into the cavity of the reverse mold. After the flexible and resilient material cures to form a positive flexible pattern of the impeller, the positive flexible pattern is removed from the flexible negative mold to form a positive flexible pattern of the impeller.
The flexible pattern is then placed in an open top metal flask, and foundry plaster is poured into the flask. After the plaster has set up, the positive flexible pattern is removed from the plaster, leaving a negative plaster mold. The flask is removed from the plaster mold which is dried to remove moisture, and a non-ferrous molten material is poured into the plaster mold. After the non-ferrous molten material solidifies and cools, the plaster is removed and destroyed to produce a positive non-ferrous reproduction of the original part or impeller. This casting process is used for non-ferrous or lower temperature casting materials and cannot be used for producing parts of high temperature casting materials such as ferrous metals and titanium.